News and Events in the Community

Posted on: February 23, 2017

Alto Tunnel Study Project Update and Notice of Chapman Drive Closure at Corte Madera Avenue

Alto tunnel study update

February 22, 2017

Dear Alto Tunnel Study interested community member,

We are providing you an update on the County of Marin’s geotechnical investigation of the Alto Tunnel, which is currently underway.  This email provides general information about the effort, what has been accomplished to date and the current schedule moving forward, weather permitting.  More information is available about the effort on the Alto Tunnel Study webpage at

Progress to date

At the end of January, the project team’s environmental consultants did an assessment of any special-status or sensitive plants or animals that could be disturbed within 250 feet of either the Underhill or Chapman sites.  These biologists found that neither the Underhill nor the Chapman site provides habitat for special-status species except nesting migratory birds; however, no such birds were observed during a nesting bird survey conducted on Jan. 30, 2017.  Therefore, planned drilling activities should not result in any potentially significant adverse biological impacts to the environment.

Drilling preparations began at the Underhill site on Feb. 1, but were quickly delayed by intense storms.  The drilling contractor installed additional erosion protection, and avoided drilling most of last week as a precaution.  They have resumed drilling and will continue to drill this week, unless weather again delays them.  Weekly progress reports will be posted to the webpage.

Revised schedule moving forward

The team has been able to drill much of this week, but the original schedule has been impacted by the last two week’s rains.  At this point, it is likely that the Underhill drilling and demobilization will not be complete until later this week.  No detours on Chapman will occur until work at Underhill is complete.  

After the contractor has finished drilling and has sealed the two boreholes at the Underhill site, they will move their equipment to the Chapman site.  After about two days of set-up, they anticipate taking about 15 working days to drill the three holes at that site.  Chapman Drive will be closed at Corte Madera Avenue during this whole period.  All Chapman Drive residences will be accessible from the north.  The expected date for closure of Chapman Drive at Corte Madera Avenue is March 1, 2017. The closure will be from March 1st through March 27th, but please always refer to the Alto Tunnel Study webpage for the most up-to-date schedule and other information regarding this phase of the work.

Once the five holes at the two sites have been bored, a scanning device will be sent down each hole to capture laser images of the tunnel interior.  This work will take a total of two days.  After the Chapman scans are done, all equipment will be moved off of that site and the Chapman detour will be lifted.  Once the tunnel scans have been evaluated, a draft study will be released and presented to the public later this summer.

Background information on the Alto Tunnel Study


The purpose of this geotechnical and structural investigation phase of the Alto Tunnel study is to observe the interior structural condition of the Alto Tunnel.  This information is necessary to better identify the scope of improvements that would be required to reopen the Alto Tunnel as considered in the 2010 Mill Valley/Corte Madera Corridor Study and to update previous cost estimates.  Today’s Alto Tunnel study is just the next step toward developing more reliable information regarding the overall improvements and costs that would be required to reopen the tunnel. The draft study’s results will be presented to you and to the greater public in summer 2017.

The investigative study

This effort is an investigative study, not a construction project.  The condition of the tunnel will be observed using remote scanners and laser mapping technology, thereby avoiding the higher cost and impacts associated with temporarily re-opening a portal and physically entering the tunnel.  This phase of work consists of drilling five holes (each approximately four inches in diameter) from two locations above the Alto Tunnel, then lowering a laser scanning probe into the holes, which will collect laser scans of the tunnel. Drilling is planned at the following two locations, which were chosen to minimize the number of holes and to reduce the disruption to area residents and to traffic, particularly during work and school commute periods:

•         In the vacant County-owned lot south of 35 Underhill Road (just south of Upperhill Road)

•         Adjacent to Chapman Drive at Camino Alto.  

Alto Tunnel Study Field Investigation Site Area Map

(See attachment for map)


To ensure drilling accuracy from one day to the next, it is necessary to keep drilling equipment properly aligned. Traffic at the Underhill Road site will not be impacted because all work will be done off of the roadway.  Due to the angle of the drilling boom and the narrow width of Chapman Dr., closure to vehicular traffic is necessary.

Expect a road closure at the top of Chapman Dr. at Corte Madera Ave. during the drilling phase (approximately March 1 through March 27, 2017, subject to change) with a detour/alternative route to lower Chapman Dr.  This temporary vehicular closure will be in coordination with the Town and emergency response officials.  Notification to residents and the community will be given in advance to alert travelers of the change and to communicate alternative routes to Chapman Dr. residences, which will be from the north during this period. Please see attachment for map. 

Please contact Carey Lando at or 415-473-5078, or the County’s outreach consultant, Victoria Eisen of Eisen|Letunic, at or 510-525-0220, with further questions.

Link to Alto Tunnel Study Project Update
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